“Is it a predictor of future housing prices? The graph that Rich posted shows Median going up but Case-Shiller going down.”
It shows what many people view as the most accurate accounting of the actual historical position and trend in housing prices.

“So does it mean that the Case-Shiller index “unmasks” the median indicator and shows the real price decrease?”

It is not affected by the randomness in trends of house sales prices. It adjust for the actual composition the houses sold and other factors,I believe.